Plasma membrane provides an interface between cell and its environment that can sense and perceive physical and chemical signals. To understand the nature of signaling mechanism in the membrane lipid bilayer, it is of primary importance to know the protein -protein interactions and the effect of physico-chemical nature of the lipid bilayer on these interactions. Specifically, here we propose to study the effects of the composition of the lipid bilayer and the 'fluidity' of the bilayer on interactions or coupling of the signaling proteins in the bilayer. We will use a rapid kinetic flow cytometer , a unique instrument at LANL, to study some of these interactions. The studies proposed here will allow us to measure the intermolecular interactions of signaling molecules and the lipid bilayer effects on these interactions in real time, quantitatively.